1. Field of the Invention
This invention relates to improving the rebound characteristics of a bat for baseball or softball.
2. Description of the Related Art
Conventionally, the flying distance of a ball is thought to be extended when hit by a high strength bat. This is because it was considered that a bat with a high degree of strength has excellent rebound characteristics and the flying distance of a ball depended solely on these rebound characteristics of the bat. However, in recent years, it is understood that a bat which can reduce the consumption of impact energy as ball transformation energy when a ball and bat collide, can extend the flying distance of a ball. In addition, it is understood that the higher the rebound force/power of the bat itself, the further the flying distance of a ball is extended. Thus, as shown in Japanese Laid Open Patent 2001-79131, a double layered bat formed by arranging a further thin metal tube (called “exterior tube” and “interior tube” respectively) on the exterior or the interior of the body of a metal bat is proposed. That is, a crevice is arranged between the bat body and the exterior tube or the bat body and the interior tube. In this way, an attempt is made to extend the flying distance of a ball by reducing the consumption of impact energy as ball transformation energy by bending of a bat's thin tube when a ball is hit.
FIG. 11 is a longitudinal direction notch sectional view and a cross sectional view of the line A-A′ in a width direction of one example of a double layered bat. The bat in FIG. 11 is comprised from a bat tip part 1, a bat impact part 2, a bat taper part 3 and a bat grip part 4. An exterior tube 20 fits onto the bat from the bat tip part 1 up to the bat is taper part 3. In the bat taper part 3, the exterior tube 20 is latched to a hollow for latching 70 which is arranged on the bat by a swaging process or spinning process. At this time, the exterior tube 20 is latched so that a fixed crevice is arranged between the bat and the exterior tube 20. Here the grip part 4 is the part of the bat which is held by a batter and is the thinnest part of the bat which extends to a predetermined length from the end of the grip. The taper part 3 is the part which has a diameter which gradually becomes larger from the grip part 4. In addition, the bat tip part 1 is the part which is formed by a resin for example, and is the end of the bat located in the furthest position from the grip end. The bat impact part 2 is the part except the bat tip part 1, the bat taper part 3 and the bat grip part 4, and which extends to a predetermined length towards the bat tip part 1 from the widest part of the taper part 3 part (in the case of a double layered bat, both an exterior tube and an interior tube of a double layered structure located at the part which extends to a predetermined length towards the bat tip part 1 from the widest part of the taper part 3 part). The bat impact part 2 is generally the part which is suitable for hitting a ball. Furthermore, depending on the manufacturing method, the tip part is sometimes formed as a part of the bat impact part 2 from metal and in this case, the bat is comprised of the bat impact part 2, the bat taper part 3 and the bat grip part 4.
This double layered bat requires latching two tubes during final processing by a swaging process for example. In this case, in order to remove processing distortions, a heat treatment is performed after swaging processing and stress must be released. However, control of a heat treatment of a part having a double layered structure is difficult and securing sufficient strength and durability is also difficult. Particularly, in the case where the material of the bat body and exterior tube or bat body and interior tube are different, because the melting point of each material is also different, control of the temperature and time of a heat treatment becomes complex.
However, as another approach, a recess part is arranged in the metal bat and a layer of urethane is formed on this recess part, as shown in Japanese Laid Open Patent 2005-305146. In the present specification, this type of bat is called a hybrid type double layered bat. FIG. 12 is a longitudinal direction notch sectional view and a cross sectional view of the line B-B′ in a width direction of one example of a hybrid type double layered bat. In FIG. 12, the diameter of the bat impact part 2 of the bat body 10 is reduced smoothly by a swaging process for example, from a boundary with the bat tip part 1 and from a boundary with the bat taper part 3 in the direction of the interior of the bat impact part 2. Then, a flexible layer of urethane 101 is formed to cover/enclose the bat body 10 and a hard layer of urethane 102 is formed on the exterior surface. In this hybrid type bat, when a ball collides with the bat, the two layers of urethane (101 and 102) contract and consumption of the impact energy as ball transformation energy is reduced as much as possible and the flying distance of the ball can be extended. However, because an urethane layer deteriorates by secular distortion, the durability of the bat is inferior compared to a metal manufactured bat. In addition, the bat also suffered from an increase in miss hits.
Thus, as shown in Japanese Laid Open Patent 2008-29620, a double layered bat was manufactured using a fixing component.